J. Nannoplankton Res., 39(1), 2021, pp. 15–28 © 2021 International Nannoplankton Association
ISSN 1210-8049 Printed by The Sheridan Press, USA

15

Miocene to Recent calcareous nannofossil biostratigraphy inthe eastern Bengal Fan (Indian Ocean): Linking turbidites
to tectonic activity during the evolution of the Himalayas
Jarrett W. Cruz*, Sherwood W. Wise Jr., William C. Parker
Department of Earth, Ocean and Atmospheric Sciences, Florida State University, Tallahassee, FL 32306, USA; Jwc09e@my.fsu.edu
Manuscript received 21st April, 2020; revised manuscript accepted 25th April, 2021

Abstract The geological history of the Himalayas and the Tibetan Plateau is currently being investigated by abroad spectrum of researchers. This paper presents the biostratigraphy of calcareous nannofossils from IODP Expedition 354, Site 1451 in the Bengal Fan. Detailed nannofossil distribution data from the Holocene (0 Ma) to the
Miocene/Oligocene boundary (23 Ma) were recorded from Site 1451. Here, we utilised their biostratigraphic ages toanalyse events captured in the cores that could be linked to the evolution of the Himalayas. The Gradstein et al. (2012)
Geologic Time Scale was used to assign ages to the Martini (1971) and Okada & Bukry (1980) biozone markers.
Fifty-six biostratigraphic events were observed in Site 1451, based on the distribution of 45 marker species, and thesewere used to interpret 51 biozones/subzones. A total of 245.1 m of turbiditic sediments were cored, from 16 to 1085.8
mbsf. This accounted for 35.9% of the total drilled sediment. The lowermost turbidite sequence was assigned an age
range of 15.8 to 17.95 Ma. This study delineated at least seven pulses of turbiditic activity in this region. Two significant pulses were correlated with climatic and tectonic forcing, which are discussed as plausible source mechanisms.

Keywords nannofossils, Himalayas, Indian Ocean, evolution, turbidites, tectonics, climate forcing

1. Introduction
The Himalayas are home to some of the highest mountains
in the world, with Mount Everest, located in Nepal/Tibet,
being the highest mountain above sea level. The buildingof the Himalayas had direct effects on climatic forcing,playing a role in the evolution of the local palaeoenvironment. The geological history of this area would be expected to contain evidence of such things as monsoon winds,
glacial and interglacial cycles, increased sedimentary erosion and shifting drainage systems facilitating sedimenttransport. While the Himalayas have attracted a great dealof investigation, a problem resides in the broadness of theevidence and in how that evidence is interpreted. Worksdone by authors such as Zhisheng et al. (2001), Royden
et al. (2008), Wang et al. (2012) and Curray (2014) havegiven us a timeline for this evolution.
The evolution of the drainage systems transporting
materials from the continental Himalayas to the surrounding ocean is reflected in the sediment record, providing
evidence of periods when intense sedimentation took
place, with the location of deposition reflecting the activity of the various drainage systems. Allen (2005), Clift &Blusztajn (2005), Curray (2014) and Blum et al. (2018)
have discussed aspects of Himalayan deposition, looking
into the evolution and reorganisation of the Himalayan
and Tibetan river systems. These sedimentary records canbe analysed to create a source-to-sink model that can belinked to tectonic and climatic events.
Despite the rigour of these prior works, the timing of
the tectonic and climatic forcing events have not always
been well constrained. Our interest in the Himalayas andTibetan Plateau was focused on the depositional characteristics of the Bengal Fan (Indian Ocean), with the goal ofusing calcareous nannofossils to develop a local biostratigraphic zonation scheme for a portion of this. We foundstrong autogenic signalling from the sediments originatingin the Himalayas and Tibetan Plateau. The timing of these
events can be used to test various hypotheses constructedto constrain the timing of the evolution of the Himalayas.
2. Location and methods
IODP Site 1451 was drilled in the Indian Ocean, in the eastern portion of the head of the Bengal Fan, at 8°0.4203’N,
88°44.4745’E (Figure 1). The water depth was around3607 m to the sea floor. Hole 1451A was drilled from thesea floor to 578.5 mbsf. There was 86% recovery in thecored intervals, resulting in 497.51 m of sediment in total.
Hole 1451B was drilled from 542 to 1181.3 mbsf, with
29% of the cored interval being recovered, giving a totalof 185.4 m. There was a 35-m overlap between the base ofHole A and the top of Hole B. The lithology was variablethroughout the cores. All the lithological units below 700

16

Cruz, Wise, Parker

and comparison using BugCad software(BugWare, Inc.).
The standard nannofossil biozonations of Martini (1971), Bukry (1973)
and Okada & Bukry (1980) were used todetermine the nannofossil age datums.
In addition, Young (1998) and Denne etal. (2005) were employed to assign ages
to the secondary events. BugWin andNannoware software, and the Nannotax
website (www.nannotax.org), were consulted to help identify the nannofossil
species.
3. Biostratigraphy of Site
1451
Sixty-five global and regionally recognised nannofossil marker species, from
Martini (1971), Bukry (1973), Okada &Bukry (1980), Young (1998) and Denneet al. (2005), were used to determine thebiostratigraphy of Site 1451, with 61
bioevents used to assign biozones andsubzones (Table 1, Plates 1–5).
Figure 1: Location of IODP Site 1451, Indian Ocean. After France-Lanord et al. (2016)
An age/depth analysis of the newdata from the post-cruise samples
mbsf were lithified.
showed significant differences from the3
Samples (20 cm ) were collected from each 1.5-m sec- shipboard study. The new model (Figure 2) is included
tion of the cores and vacuum-sealed in bags. High-reso- here to provide a basic idea of the age/depth model for
lution sampling (one sample per 10 mm) was performed IODP Expedition 354, Site 1451. Figure 2 was created usupon visual observation of a change in lithology, and ing the new biostratigraphic datapoints, along with data
samples were taken from above, below and within turbi- from Blum et al. (2018). It also shows the core recovery
dite units. High-resolution samples were also taken from from the two holes from Expedition 354 and the lithologithe pelagic lithologies. A total of 420 samples were col- cal units they were assigned to.
lected and examined for nannofossils. The standard smearslide technique was used for sample preparation (Bown & 4. Turbidite activity at Site 1451
Young, 1998).
During the expedition, as drilling was taking place and
The slides were examined using Zeiss Standard 25 the cores were being processed, a team of sedimentoloand Zeiss Axiostar Plus light-microscopes. Counts were gists analysed the cores, detailing the lithological variaobtained from one traverse, viewing the samples at 1000x tions and the presence of turbidite units. There were 245.1
magnification (~120 fields of view). This allowed for a
semiquantitative count of the species abundances. To find Table 1 (right): Nannofossil biostratigraphy of IODP Expedition 354,
rare specimens, another three traverses were observed. Site 1451. Zones based on marker events assigned by Martini (1971,
BugWin counting software (BugWare, Inc., Tallahassee, NP/NN nomenclature), in addition to Bukry (1973) and Okada & Bukry(1980, CP/CN nomenclature). Ages were calibrated based on Gradstein
FL, USA) was used on all 420 samples in order to compile
et al. (2012), plus one age from Young (1998) and six from Denne et al.
distribution data that was then plotted for detailed analysis (2005)

Miocene to Recent biostratigraphy, turbidites and tectonics, eastern Bengal Fan
Depth

SamplesZone(m)

IODP core ID

Martini (1971)

3.67

A-1H-3, 66–67

NN21

Zone
Okada & Bukry(1980)
CN15

Age

Markers

Gradstein et al. (2012) (Ma)

Top/Base/X

RecentE. huxleyi

0.29

Gephyrocapsa spp. andabsence of E. huxleyi
Top P. lacunosa

29.6

A-5H-1W, 74–75

NN21/NN20

CN14b/CN15

64.38

A-11F-3W, 108–109

NN20/NN19

CN14a/CN14b

0.44

104

A-20F-1, 142–143

NN19

CN13b

1.26

Top H. sellii

106.2

A-20F-3, 64–65

NN19

CN13b

1.6

Top C. macintyrei

110.1

A-21F-2, 131–132

NN19/NN18

CN12d/CN13a

1.93

Top D. brouweri

122.5

A-23G-1, 100–101

NN18

CN12d

1.95

Top D. triradiatus

123.6

A-23H-2, 67–68

NN18/NN17CN12c/CN12d

2.39

Top D. pentaradiatus

137.3

A-24H-5, 125–126

NN17/NN16

CN12b/CN12c

2.49

142.6

A-25H-2, 63–64

NN16

CN12a/CN12b

2.8

144.3

A-25H-3, 78–79

NN16

CN12a

3.54

148.2

A-25H-6, 22–23

NN16/NN15

CN11b/CN12a

3.7

Top D. surculus
Top D. tamalis (firstcommon)Top Sphenolithus spp.
(first common)
R. pseudoumbilicus

151.8

A-26H-2, 41–42

NN15/NN14

CN11b

3.92

152.6

A-26H-3, 65–66

NN14/NN13

CN11a/CN11b

4.13

153.9

A-26H-4, 84–85

NN13CN10c/ CN11a

4.5

161.2

A-27H-2, 44–45

NN13

CN10c

5.04

Top C. acutus

162.7

A-27H-3, 55–56

NN13/NN12CN10b/CN10c

5.12

Base C. rugosus

A. tricorniculatus
Base common D.
asymmetricus
A. primus

165

A-27H-5, 29–30

NN12

CN10b

5.28

Top T. rugosus

169.6

A-28H-1, 60–61

NN12

CN10a/ CN10b

5.35

Base C. acutus

171.1

A-28H-2, 60–61

NN12/NN11

CN9d/CN10a

174.1

A-28H-4, 60–61

NN11

183

A-29H-5, 32–33

NN11

189

A-30H-2, 60–61

NN11

255.3

A-43F-1, 80–81

NN11

CN9c

6.79

259.1

A-43F-4, 45–46

NN11

CN9b/CN9c

301

A-52F-3, 78–79

NN11

CN9a/CN9b

345.8

A-62F-1, 59–60

NN11

CN9a

7.53

Top D. loeblichii

8.29

Base D. berggrenii/ Top
D. prepentaradiatus

346.4

A-62F-2, 15–16

NN11/NN10CN9c/CN9d

CN8/CN9a5.59

Top D. quinqueramus

5.59 (Young, 1998)

Top C. mediaperforatus

5.94

Top N. amplificus

6.1 (Denne et al., 2005)

6.91

Top R. rotaria
X N. amplificus to T.
rugosusBase of N. amplificus

7.42

Base A. primus/spp.

373.4

A-70F-1, 65–66

NN10

CN8

8.68

Top M. convallis

440.6

A-82F-1, 86–87

NN10

CN8

8.77

Base D. loeblichii

441.6

A-82F-2, 19–20

NN10

CN8

9.21

Top D. bollii

442.5

A-82F-3, 11–12

NN10/NN9

CN7/CN8

9.53

Top D. hamatus

449.7

A-86F-1, 65–66

NN9

CN7

9.69

Top C. coalitus

584.95B-6X-4, 56–57

NN9/NN8

CN6/CN7

10.55

Base D. hamatus
Base D. brouweri

590.9

B-6X-6, 27–28

NN8

CN6

10.76

620.2

B-11X-1, 60–61

NN8/NN7

CN5b/CN6

10.89

Base C. coalitus

695.14

B-20R-1, 61–62

NN7

CN5b

11.21

Top C. premacintyrei

717.54

B-22R-3, 2829

NN7

CN5b

11.85

787.96

B-29R-4, 58–59

NN7/NN6

CN5a/CN5b

11.9

794.53

B-30R-2, 84–85

NN6

CN5a

12.12

831.41

B-34R-1, 44–45

NN6

CN5a

12.83

831.41

B-34R-1, 44–45

NN6

CN5a

13.1 (Denne et al., 2005)

845

B-36R-1, 35–36

NN6

CN5a

13.28

Top C. floridanus
Base common D.
kugleri
Top C. nitescens
Base R.
pseudoumbilicus
Top D. virginianus
Top common C.
floridanus
Top S. heteromorphus

872.3

B-38R-2, 119–120

NN6/NN5

CN4/CN5a13.53

899.61

B-41R-1, 51–52

NN5

CN4

14.1 (Denne et al., 2005)

Top D. petaliformis

948.88

B-46R-1, 108–109

NN5/NN4

CN3/CN4

14.91

Top H. ampliaperta
Abundant D. deflandrei
group
Top S. belemnos

954.23

B-47R-3, 2–3

NN4

CN3

15.8

1096.93

B-63R-3, 33–34

NN4/NN3

CN2/CN3

17.95

1098.35

B-63R-4, 81–82

NN3/NN2

CN1c/CN2

18.28

Top T. carinatus1114.75

B-65R-2, 40–41

NN2

CN1c

20.8 (Denne et al., 2005)

Top S. dissimilisBase S. disbelemnos

1127.61

B-67R-4, 71–72

NN2

CN1c

22.76

1130.56

B-67R-6, 84–85

NN1

CN1a–b

23.11

Top S. delphix

1136

B-68R-4, 0–0

NN1/NP25

CP19b/CN1a–b

23.13

Top R. bisecta

17

18

Cruz, Wise, Parker

Figure 2: Age/depth plot for IODP Site 1451, including data from Blum et al. (2018) and the new nannofossil datapoints (this study, in blue). The red
datapoints are foraminiferal age constraints and the green datapoints are radiometric age constraints (after Blum et al., 2018). The lithological units
are those interpreted during the expedition

Miocene to Recent biostratigraphy, turbidites and tectonics, eastern Bengal Fanm of core listed as silt, sand or turbidite, accounting for35.9% of the total recovered sediment. From these descriptions, we listed the identified turbidite units (Table 2).
We assigned ages to each unit based on the nannofossil
markers identified from above, below and within the units.
Reworked Palaeogene nannofossils were consistently observed; however, we had high confidence in the biostratigraphy due to the Neogene marker-species present in the
pelagic and hemipelagic sediments.
An important goal for Site 1451 was to identify thelowermost turbidite unit; the lowest sequence was observed in Core 62R, its base being at 1085.8 m. An ageof 15.8 to 17.95 Ma was assigned to the unit. Based on
sedimentological observations and nannofossil ages, there
appeared to be at least seven pulses of turbidite activity
present in the cores.
The turbidite units were produced by turbidity currents, which are a form of sediment gravity flow wheresediment grain support is generated by turbulence. The
stronger the turbidity flow, the larger the grain size of thesediments it can carry. Coarser sediment grains are carried in the bed near the front of the flow. These currents
are responsible for creating canyons, channels and lobes
in the world’s oceans. When a turbidity current deposits
turbidite units, it usually follows the Bouma sequence oflithologies. Coarser materials are deposited at the base,with the grain size grading to finer particles towards the
top in a layer-stacking event that is usually repeated, sequence upon sequence. Layers can be lost or compressed
due to mixing or settling.
In the observed turbiditic units, seven events wereidentified when split by nannofossil zone. An Early Miocene event was recorded between Cores 354-1451B-49R
and 354-1451B-62R (967.27–1086.47 m). There were 9.2
m of turbiditic lithology recorded. This event was in NN4(17.95–15.8 Ma) and represented the earliest recorded turbidite at Site 1451.
The second oldest event was during the Middle Miocene, recorded between Cores 354-1451B-41R and354-1451B-45R (899.1–940.44 m). There were 7.49 mof turbiditic section in NN5 (~15.8–14.1 Ma). The third
event was in 354-1451B-29R to 354-1451B-37R (785.91–
863.31 m), where 13.99 m of turbidites were observed in
NN6 (~13.53–11.9 Ma). The fourth event was recorded
in 351-1451B-13X to 354-1451B-29R (639–785.86 m).
Here, there were 20.7 m of turbiditic section in the NN7/19

NN8 transition and NN7 (~11.85–10.89 Ma).
The other three events occurred in Upper Miocene
sediments, the fifth recorded between 351-1451A-84F
and 354-1451B-6X (449.3–581.72 m), totalling 29.06
m. This event was in NN9 (10.55–9.69 Ma). In NN7–9(10.97–9.69 Ma), there were 36.46 m of turbidites, averaging 28.07 m/Myr––almost double the average depositional rate at the site (i.e. 14.58 m/Myr). The sixth event
was in 354-1451A-66F to 354-1451A-82F (363.7–441.2m), with 24.52 m of turbidites in the NN10/NN11 transition and NN10 (9.21–8.29 Ma). The seventh event wasrecorded between 351-1451A-28H and 354-1451A-60F(174.45–339.99 m). There were 59.44 m of turbidites, falling in NN11 (~8.29–5.59 Ma). From 8.29–6.1 Ma, there
were 58.14 m of turbidite sediments reported. This was
26.55 m/Myr, also nearly double the recorded average (i.e.
14.58 m/Myr).
5. Discussion
5.1 Early Miocene uplift of the Tibetan
Plateau and the initiation of monsoons
The Early Miocene evolution of the Himalayas and the Tibetan region saw a major shift in the palaeoenvironment.
This is thought to be the timing of the initiation of monsoons in the region as a result of significant tectonic forcing. The Early Miocene was characterised by long-term
sea-level highstands and the Early to Middle Miocene Climatic Optimum (Betzler et al., 2018). This coincided withcrustal thickening and rapid surface uplift in the Tibetan
Plateau. There was active eastward flow of deep crust,
while southward thrusting was taking place in the Lesser
Himalayas (Royden et al., 2008). This significant tectonic
activity, coupled with the onset of monsoons, thought tohave initiated around 20–15 Ma (Clift et al., 2008), generated copious amounts of clastic material, which wastransported south into the Bay of Bengal. This correlateswith the initiation of turbidite deposition at Site 1451. In
NN4, between 17.95 and 15.8 Ma, 9.2 m of turbidites
were preserved in Cores 354-1451B-49R to 354-1451B62R (967.27–1086.47 m), and these contained significant
plant-fragment inclusions; in fact, 13.7 m of sediments
containing plant-fragment inclusions were recorded from
Site 1451 in this time interval (Cruz, 2020).
Around 20–15 Ma, rapid erosion and transport of sediment from the Himalayas (Clift et al., 2008; Blum et al.,2018) resulted in deposition in the Bay of Bengal via the20

Cruz, Wise, Parker

Core

354-U1451A-3H
354-U1451A-4H

354-U1451A-5H

354-U1451A-6H
354-U1451A-7H
354-U1451A-8H
354-U1451A-9H
354-U1451A-10H
354-U1451A-12H
354-U1451A-13H
354-U1451A-14H
354-U1451A-16F

Start
depth
(m)
16
24
25.77
27.44
28.9
29.73
30.4
34.75
39.7
42.96
46.3
52.3
55.6

354-U1451A-19F

67.74
69.7
74.4
83.92
85.42
85.64
85.7
86.8
88.58
92.48
97.9

354-U1451A-22H

112

354-U1451A-28H

174.45174.82175.3
180.84354-U1451A-17F

354-U1451A-29H

354-U1451A-30H

354-U1451A-31H
354-U1451A-33F
354-U1451A-37F
354-U1451A-39F
354-U1451A-41F
354-U1451A-45F
354-U1451A-47F
354-U1451A-49F

189.86
191.67
197.5
199
197.5
207
226
235.5
245
264
273.5
283

354-U1451A-54F
354-U1451A-56F
354-U1451A-58F
354-U1451A-60F

306.7
316.2
325.7
335.2

354-U1451A-66F

363.7

354-U1451A-68F
354-U1451A-70F
354-U1451A-72F
354-U1451A-74F
354-U1451A-76F
354-U1451A-78F
354-U1451A-80F
354-U1451A-82F

374.7
384.61392.23
401.7
411.2
420.7
430.2
439.78354-U1451A-31F

End
Martini
GTS2012depth(1971) Zone Age (Ma)(m)
Recent
23.62
age
25.23
27
NN21
28.93
29.56
NN21/NN20
0.29
29.89
34.75
39.75
42.96
NN20
46.33
52.38
55.56
60.74
NN20/NN19
0.44
69.76
74.3
79.39
85.19
85.47
85.68
NN19
86.7
88.66
88.68
93.45
102.85
NN19/NN18
1.93
118.37NN18
NN12/NN11
5.59
174.62174.89175.89181.31
5.94
6.1
191.36
197.19198.88201.63198.81211.79230.65NN11240.34
246.64268.99277.37285.687.42
311.51321.03329.61
339.99NN11/NN10
8.29
366.768.68
375.69385.16397.11NN10
405.94415.08421.63434.77441.2
NN10/NN99.53

Turbidite
units

Core
354-U1451A-84F
354-U1451A-86F
354-U1451A-88F
354-U1451A-90F
354-U1451A-92F
354-U1451A-94F
354-U1451A-96F
354-U1451A-98F
354-U1451A-102F
354-U1451A-104F
354-U1451A-106F
354-U1451A-111X
354-1451B-3X
354-1451B-4X
354-1451B-5X
354-1451B-6X

Start
depth
(m)
449.3
458.7
468.26
469.6
477.7
487.2
496.7
506.2
515.7
534.7
544.2
553.7
576.12551.82561.4
571.1
580.93

End
depth
(m)
452.11462.09469.29471.11479.19489.01497.76507.55518.38537.91545.18555
576.54556.27561.68571.6
581.72354-1451B-10X

610

610.1

354-1451B-13X
354-1451B-16R
354-1451B-18R
354-1451B-19R

639
655.6
675.2
685
687.66688.09694.8
696.69697.2
697.35697.72704.78705.6
705.68705.89714.4

639.29658.27676.3
687.63687.92688.44695.47696.88697.27697.43697.86705.33705.68705.8
706.1
717.25

734
763.2
772.8
782.5
784.94785.91

734.19766.42775.15784.26785.86787.43354-1451B-32R
354-1451B-33R
354-1451B-34R

788.1
792.23795.19
811.6
821.3
831

788.51795.19796.75812.55823.12
832.73354-1451B-37R

860.27863.31354-1451B-41R
354-1451B-42R
354-1451B-43R
354-1451B-45R

899.1
908.9
918.6
938

900.77910.32920.56940.44354-1451B-20R

354-1451B-21R

354-1451B-22R

Martini(1971)
Zone

GTS2012Age (Ma)

Turbiditeunits

9.53/9.69

NN95

NN9/NN8
NN8

10.55
10.76

NN8/NN7

10.89

NN7

11.21

4

11.85

7

354-1451B-24R
354-1451B-27R
354-1451B-28R
354-1451B-29R

NN7/NN6354-1451B-30R

12.12
NN6

NN5/NN4354-1451B-53R
354-1451B-54R
354-1451B-55R
354-1451B-56R
354-1451B-59R
354-1451B-61R
354-1451B-62R

970.13997.25999.341006.311016.171018.26
1028.291037.21058.4
1074.81086.47

13.53
14.1

NN5

6
967.27996.4
998.061006.161015.81016.41025.61035.31054.81074.21083.93
12.83-13.1
13.28

NN6/NN5354-1451B-49R
354-1451B-52R

11.9

2

14.91
15.8

NN4

1

17.95

Table 2: Turbiditic sequences found during IODP Expedition 354, Site 1451

Miocene to Recent biostratigraphy, turbidites and tectonics, eastern Bengal Fan

Ganges River system. By 18 Ma, the drainage of Tibet
via the Ganges was at least partially integrated with thatof the Brahmaputra River, and significant uplift in the Tibetan region would have caused an increase in sedimenttransport through the Brahmaputra River system into the
Bengal Fan.
5.2 Middle Miocene climatic forcing
The early Middle Miocene includes the Middle Miocene
Cooling Event, a time when palaeotemperatures indicate
a significant drop in temperature coincident with a longterm sea-level lowstand. The eastern Antarctic ice-sheet
expanded, causing this decrease in sea level, at around15.1 Ma (Betzler et al., 2018), and this climatic driver
likely contributed to the increased deposition of turbidites
at Site 1451. This increase was recorded between Cores
354-1451B-41R and 354-1451B-45R (899.1–940.44 m),with 7.49 m of turbidites occurring in NN5 (15.8–14.1
Ma). There was also a shift in plant-fragment deposition
during this interval. At around 14.91 to 14.1 Ma, there
were 3.08 m of plant-fragment inclusions in the turbidite deposits. However, before that, from 15.8–14.91 Ma,
there was no turbidite deposition. Platform growth in theMaldives at this time saw a shift from aggradation to progradation, as noted by Betzler et al. (2018), while at Site
1451, there was a shift from turbidite deposition to 30 m ofpelagic/hemipelagic deposition (i.e. lasting for 890 kyr).
In the late Middle Miocene, the monsoon winds in the
Indian Ocean strengthened (Betzler et al., 2018). At thistime, there was a change in sedimentation patterns, driven
by these shifting monsoon winds, as well as alterations tothe circulation in the Indian Ocean. The hypothesised timing of this strengthening coincided with two significant
pulses in turbidite deposition and an increase in plantfragment inclusions. The first of these pulses was found inCores 354-1451B-29R to 354-1451B-37R (785.91–863.31m), with 13.99 m of turbidites dated as NN6 (13.53–11.9
Ma). The second was recorded between 351-1451B-13X
and 354-1451B-29R (639–785.86 m), with 20.7 m of turbidites dated as NN7/NN8 and NN7 (11.85–10.89 Ma).
Betzler et al. (2018) determined an increase in organicmatter content in deposits lying above the shift in depositional patterns. This can be seen at Site 1451, between
11.9 and 10.89 Ma, where there were 10.22 m of section
containing plant-fragment inclusions, compared to 2.27 mfrom 12.83 to 11.9 Ma. This increase in deposition and21

plant fragment inclusion supports a strengthening of themonsoon winds, as hypothesised by Betzler et al. (2018).
5.3 Late Miocene tectonic and climatic
forcingWang et al. (2012) suggested that the Himalayas underwent tectonic forcing around 7+ Ma, uplifting the regionby 2–2.5 km to its current elevation of 4100–4500 masl.
This timing of central Himalayan uplift is supported by theincrease in turbidite deposition at Site 1451 between cores351-1451A-28H and 354-1451A-60F (174.45–339.99 m),
where 59.44 m of turbidites were deposited, which accountfor 24.25% of the total turbidite deposition at the site. Thisdepositional event is in NN11, and occurred from 8.29 to
5.59 Ma. There was a significant pulse around 7.42–6.1
Ma, demarcated by the top occurrence of Reticulofenestra
rotaria and the base occurrence of Amaurolithus primus.
Of the 157.4 m of sediments deposited in this interval,
55.08 m were turbidites. The sedimentation rate during
this time was high, at 26.55 m/Myr.
Wang et al. (2012) showed a similar increase in sedimentation rate, to 60±22 mm/kyr, in the Gyirong Basin(Tibet), where sediment accumulated between 10.2 and7.2 Ma. Simultaneously (8.29–6.1 Ma), transported sedimentation increased at Site 1451. This supports significant
uplift occurring in the central Himalayas around 7.42+ Ma.
Two other events occurred during the Late Miocenethat can be attributed to climatic forcing in eastern Tibet.
Zhishent et al.’s (2001) hypothesis that enhanced aridity in
the Asian interior, along with the onset of strengthening in
the Indian monsoon, was initiated around 9–8 Ma is supported by the increase in sedimentation rates during thisperiod. This would have increased erosion and the transport of materials from the continental shelf into the Bay of
Bengal. The first event was recorded between 351-1451A84F and 354-1451B-6X (449.3–581.72 m). The 29.06 mof turbiditic section (11.85% of the total turbidite deposition at site 1451) occurred in NN9 and around the NN9/NN8 boundary (~10.55–9.69 Ma). From 10.97–9.69 Ma,
there were 36.46 m of turbidite sediments, with a relatively high sedimentation rate of 28.07 m/Myr. This pulse
in turbidite deposition supports the hypothesis (Zhishent
et al., 2001) that further significant uplift in this region
may have occurred around 10–8 Ma. The second event
was recorded between 354-1451A-66F and 354-1451A82F (363.7–441.2 m). The 24.52 m of turbidites lie within

22

Cruz, Wise, Parker

the NN11/NN10 transition and NN10, at around 9.21–8.29
Ma. This accounted for 10% of the total turbidite deposition at Site 1451.

Ocean.
Acknowledgements
This work could not have been done without: samples and fund-

6. Conclusions
Biostratigraphy is a critical tool for investigating the evolutionary history of a region. In addition to providing atimeframe for the turbiditic events at Site 1451, the nannofossil dates were used in testing a number of hypotheses
from a broad array of studies. Wang et al. (2012) suggested that a major tectonic event occurred in the Himalayan
region ~7.2 Ma, which created a palaeolake basin. The
current study indicated that an uplift event was initiatedaround 7.42+ Ma in the central Himalayas. An et al. (2001)
hypothesised uplift in the Tibetan Plateau around 10–8
Ma. The current study inferred such an event to have occurred around 9.69+ Ma. Betzler et al. (2018) indicatedthat a monsoon-wind-driven model was influencing sedimentation patterns around 17–15.1 Ma. The current study
supported a strong climatic event occurring around 15.8–
14.91 Ma. Clift et al. (2005) and Royden et al. (2008) bothhypothesised an Early Miocene evolution of the Himalayas and Tibetan Plateau. At this time, around 17.95–15.8
Ma, Site 1451 had just begun receiving turbidite deposits,
confirming that the evolution of the Himalayas was wellunderway in the Early Miocene. These correlations provethat high-resolution nannofossil biostratigraphic data are
key to providing a chronostratigraphic framework for theevolution of the Himalayas.
By understanding the timing of turbidite deposition,these sediments can be linked to the forcing mechanisms
behind their formation. By doing this, an understandingof the regional and global processes that took place during the evolution of the Himalayas and Tibetan Plateau
can be obtained. Early Miocene tectonic forcing triggered
the initiation of turbidite deposition at Site 1451. This tectonic activity likely impacted the Middle Miocene climatic
forcing that increased turbidite deposition here. During
the Late Miocene, tectonic and climatic forcing increased,again altering the intensity and frequency of turbidite deposition in the Bay of Bengal. This framework provides a
backdrop for future studies, while showcasing how critical
calcareous nannofossil biostratigraphy can be in furthering our understanding of the evolution of the Himalayas,
and also in providing a high-resolution age model for addressing other issues in the Cenozoic history of the Indianing from the IODP and help from the scientists who sailed on
Expedition 354; the geology programme in the Department ofEarth, Ocean and Atmospheric Science at FSU; and BugWare,Inc. for supplying us with BugWin and BugCad. We would also
like to thank Drs David Bord, Richard Howe and Jackie Lees
for the time and effort they put into reviewing this article. All
supplementary materials can be found in Cruz, Jarrett W. 2020.
Utilizing Indian Ocean calcareous nannofossil biostratigraphyof Miocene sediments to investigate tectonic and climatic forcing
during the evolution of the Himalayas. Florida State University,
ProQuest Dissertations Publishing. 27999239. https://search.
proquest.com/openview/8b28b39dd5eb015c7d03ffee4da7afd3/
1?pq-origsite=gscholar&cbl=18750&diss=yReferences
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An, Z., Kutzbach, J.E., Prell, W.L. & Porters, S.C. 2001. Evolution of Asian monsoons and phased uplift of the Himalayan–Tibetan plateau since Late Miocene times. Nature, 411:
62–66.
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24

Cruz, Wise, Parker

Plate 1
Amaurolithus, Calcidiscus, Catinaster, Ceratolithus
5 µm

1
Amaurolithus primus

2
A. primus

5
A. tricorniculatus

3
A. primus

6

C. premacintyrei

10Catinaster coalitus

11
C. coalitus

14

13
C. sp. A. (Discoaster virginianus)

18
Ceratolithus rugosus

12
C. sp. A. (Discoaster virginianus)

15
Ceratolithus acutus

17

8

7

Calcidiscus macintyrei

94
A. tricorniculatus

19

16

Miocene to Recent biostratigraphy, turbidites and tectonics, eastern Bengal Fan

25

Plate 2
Coronacyclus, Cyclicargolithus, Discoaster
5 µm

2

1
Coronacyclus nitescens

D. bollii

9D. brouweri

D. bollii

D. brouweri

D. deﬂandrei

7

D. brouweri17

D. deﬂandrei

D. druggii

16
D. druggii

19

18
D. hamatus

12

15

14
D. deﬂandrei

8
D. bollii

11

10

13

4
Discoaster asymmetricus

65

D. druggii

3

Cyclicargolithus ﬂoridanus

D. hamatus

20
D. hamatus

26

Cruz, Wise, Parker

Plate 3
Discoaster
5 µm

1

2

Discoaster kugleri

56
D. petaliformis

9

D. pentaradiatus

7
D. petaliformis

11

10

13

12
D. quinqueradiatus

15

14

16

D. surculus

17
D. tamalis

8
D. petaliformis

D. prepentaradiatus

D. quinqueradiatus

4

3
D. loeblichii

19

18
D. tamalis

20
D. triradiatus

Miocene to Recent biostratigraphy, turbidites and tectonics, eastern Bengal Fan

27

Plate 4
Discoaster, Gephyrocapsa, Helicosphaera, Minylitha, Nicklithus
5 µm

1Discoaster triradiatus

5

D. triradiatus

Gephyrorhabdus oceanica

7

6
Helicosphaera ampliaperta

9

4

3

2
D. triradiatus

8
H. carteri

10

11

12

15

16

Minylithus convallis

13
M. convallis

14
M. convallis

17

Nicklithus amplificus

28

Cruz, Wise, Parker

Plate 5
Pseudoemiliania, Reticulofenestra, Sphenolithus, Triquetrorhabdulus
5 µm

1

2

Pseudoemiliania lacunosa

3

4

Reticulofenestra pseudoumbilicus

5

6

Sphenolithus belemnos

7
S. heteromorphus

9
S. heteromorphus

10
Triquetrorhabdulus carinatus

13

8
S. heteromorphus

11

12
T. rugosus